Quality control (“QC”) analysis of pharmaceutical products is an essential task that helps to ensure the safety of products used in the field of health care and eliminate the risk of an out of specification product making its way into a patient. In some instances, QC of a pharmaceutical product must be performed shortly after preparation of the product and immediately prior to injection into a patient. One example of a pharmaceutical product whose final preparation occurs immediately prior to injection is a hyperpolarized imaging agent for use in MRI and NMR spectroscopy.
Hyperpolarizing of an imaging agent for use in MRI and NMR spectroscopy is done to increase sensitivity in the imaging agent; however, such hyperpolarizing can only be performed immediately prior to injection of the imaging agent into a patient, as the hyperpolarized imaging agent has a very short life span. That is, the imaging agent must be quickly transferred from its production source to its place of intended end use (i.e., injection into a patient) within a matter of minutes. For such a product, QC analysis performed immediately prior to injection is the only option. Additionally, QC analysis must be performed in a short period of time, without introducing any additional chemicals to the patient, and preserving the sterility of the pharmaceutical product. Devices for such QC analysis are known and their description could be found, for example, in U.S. Pat. Nos. 7,803,320, 7,519,492, and 7,610,157.
A common hyperpolarized imaging agent is 13C1-pyruvate, as disclosed, for example, in U.S. Pat. Nos. 8,731,640, 8,763,410, and 7,633,290. In addition to pyruvate, acetate is known to be important for a number of metabolic processes. However, unlike pyruvate which can be readily measured by its ultraviolet absorption, acetate has no such absorption for QC purposes. An almost instant and strong signal from acetate agent is critical to meet 13C QC requirements. There is a known assay for acetate which utilizes an enzyme based detection method, as described, for example, in U.S. Pat. No. 4,035,239. However, enzyme based acetate assay methods are time consuming, which makes them not suitable for QC of hyperpolarized acetate due to a very short life span of hyperpolarized acetate. Additionally, enzyme based acetate assay methods are expensive. Furthermore, extra calibration steps using known acetate standards are usually required due to stability and activity difference for enzyme batches.
The literature also reports a colorimetric and fluorescent assay for acetate ions based on a rhodamine 6G-phenylurea (RGPU) conjugate in the presence of Fe(III) ions. (Hu et al., Chem. Commun., 47: 1622-1624 (2011)). In this assay, the addition of acetate ion prompts dissociation of RGPU-Fe complex and release of free RGPU, which results in change in fluorescence and color shift from pink to colorless. The reaction media of this RGPU based assay is a 1:1 mixture of water and acetonitrile and the detectable acetate concentration range is 0-200 nM. However, this concentration range is about six orders of magnitude lower than desired for QC of hyperpolarized imaging agents. In addition, this RGPU based assay will not work in 100% aqueous solution and, therefore requires multiple dilution steps to adjust acetate concentration and to adjust properties of a liquid composition containing acetate for optimal assay conditions. Therefore, the RGPU based method is cumbersome, time-consuming, and unreliable. Accordingly, there is a need for quick and inexpensive acetate assay methods which preferably use water as a solvent. The methods and complexes disclosed herein solve this need and may be used for assaying acetate in a wide variety of applications, including but not limited to assays of hyperpolarized acetate imaging agent.